Presently, microtiter plate readers with separate filter slides for excitation filters and emission filters are known. Apparatuses for irradiating samples which can be characterized by light penetrating the samples (transmission), the light reflected by the samples (reflection), light initiated on or in the sample (fluorescence) or light emitted by the samples themselves (luminescence) have long been known as microscopes, spectrophotometers, fluorometers and the like. The light that penetrates the sample during the irradiation or is reflected by the same or the fluorescence initiated on or in the sample will be designated below as “light originating from the sample” and will be measured for example by one or several photodetectors. The use of different optical filters for influencing light rays with which the samples are radiated or light that is transmitted through the same (so-called excitation filters) is known. Similarly, emission filters are known which influence the light emitted by the samples. Especially user-friendly are filter slides which comprise at least one such filter, but preferably several such optical filters, which can be moved with a defined movement into the beam path.